This invention relates to improved processes for producing activated (doped) quantum sized particles. More particularly this invention is directed to process for the production of metal oxide nanocrystalline phosphors activated with a rare earth element. The nanocrystal oxides are produced by a micellar microemulsion process which is reproducible, scalable and which provides a high yield of very small particles.
The unique physical properties of quantum sized particles (i.e. 1-10 nanometers in diameter, preferably less than 5 nm and most preferably less than 2 nm) called "nanocrystals" have attracted scientific interest. Because of their quantum size these nanocrystals exhibit properties that are far different from their bulk (larger sized) counterparts. However, the great majority of the quantum sized particles that have been studied and produced have been undoped. Doped (or "activated") quantum sized particles have unique properties which differ from non quantum sized particles of the same composition and from non-activated quantum sized particles.
In common terminology in the semiconductor art a host particle that has been "doped" generally refers to a particle that has less than 1.0% of the doping element. Despite the low percentage of the dopant the host has the properties of the doping particle. When the percentage of the dopant exceeds 1.0% the particle is generally referred to as an alloy. However, in certain applications, particularly when dealing with phosphors, the particle is referred to as doped even though it contains up to 10.0% of a "dopant" and it will act as if it were doped (ie. its properties are governed by the activator). Accordingly in this application we will refer to "activated" or "doped" host particles to include host particles which include an activator element. Thus Y.sub.2 O.sub.3 :Eu refers to a Yttrium oxide host doped or activated with Europium.
Only recently has work been done to produce doped quantum sized particles. In U.S. Pat. No. 5,522,377; issued Jun. 11, 1996 entitled "Encapsulated Quantum Sized Doped Semiconductor particles and method of manufacturing Same" there is disclosed an organometallic process for producing doped ZnS:Mn nanocrystals. However, the doped nanocrystals produced by this process have sulfide hosts. Sulfides and similar host compounds are highly reactive with air and other compounds which may render the nanocrystals unusable. Indeed, the nanocrystals produced by this process are encapsulated to reduce reactivity.
In U.S. Pat. No. 5,637,258; issued Jun. 10, 1997 entitled "Method for Producing Rare Earth Activated Metal Oxide Nanocrystals", of which one of the present inventors is a co inventor, there is disclosed a process for producing activated oxide (rather than a sulfide) nanocrystals by a "sol-gel" like process, rather than the organometallic processes previously used. However, this process can produce luminescence destroying organic byproducts which must be separated from the nanocrystals produced by the process.
The present invention is directed to a efficient, reproducible and scalable process for the production of metal oxide nanocrystals which when activated with a rare earth element are useful as phosphors. The nanocrystal oxides are produced by a microemulsion process. In the process an aqueous solution of the host and activator is prepared and added to a mixture of oil, a micelle forming surfactant and a cosurfactant to form a first water in oil microemulsion. An aqueous solution of a hydroxide containing compound is added to a second mixture of oil and a micelle forming surfactant and cosurfactant to form a second water in oil microemulsion. The two microemulsions are added together which cause the micelle units to coalesce and decoalesce and to form a nanocrystalline hydroxide compound of the host and activator. The solution is washed and treated so as to remove byproducts. Thereafter the hydroxide compound is heated or calcined to convert it to an nanocrystalline activated oxide.
The activator elements used in the present invention are the rare earth elements. In a doped or activated phosphor the characteristic light emitted by the phosphor when energized is controlled by the dopant or activator, not the host material such as yttrium oxide (Y.sub.2 O.sub.3). When used as dopants or activators in phosphors the so called "rare earth" elements have narrow spectrum light emission. A phosphor doped with Europium (Eu) will emit red light, Terbium (Tb) will emit green and Thulium (Tm) blue. Thus phosphors doped with these three elements may be used to generate a full color RGB display device. Additionally, other rare earth dopants may be used to generate emissions outside of the visible spectrum, cerium (Ce) and gadolinium (Gd) will generate ultraviolet light and Erbium (Er) will emit infrared.